Magnetic resonance imaging apparatus

ABSTRACT

A magnetic resonance imaging apparatus applied flip-pulses and gradient magnetic field pulses to generate a plurality of MR signals and to add encoding information to the MR signals, applies pre-pulses to obtain a predetermined effect, and generates an image on the basis of the MR signals. The MR signals include first MR signals arranged in a first region in a k-space and second MR signals arranged in a second region in the k-space. An applying condition for the second MR signals is differed from that for the first MR signals. The applying condition is defined as a ratio of the number of the pre-pulses to the number of the flip-pulses applied in an unit time.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is based upon and claims the benefit of priorityfrom the prior Japanese Patent Application No. 11-361298, filed Dec. 20,1999, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to a magnetic resonance diagnosticapparatus and, more particularly, to an improvement in an applicationmethod of a pre-pulse such as a fat suppression pulse applied before aflip-pulse (exciting RF pulse) is applied.

[0003] A typical fat suppression pulse as a pre-pulse reduces an MRsignal (Magnetic Resonance signal) from fat by selectively exciting onlyfat by a flip-pulse narrowed in band in accordance with a chemical shiftbetween fat and water, and then applying a gradient magnetic field(spoiler pulse) to sufficiently dephase the signal. Recently, the fatsuppression pulse is indispensable in improving the image qualityparticularly in MRA (MR Angiography).

[0004] As is well known, according to the MRA principle, a still objectsuch as a brain parenchyma or organ within the photographing region issequentially excited before the longitudinal magnetizationsatisfactorily recovers. Thus, the signal level gradually decreases.However, fresh blood (water) always flows into the photographing region,so its signal level hardly decreases in comparison with still objects.As a result, an image (bloodstream image) in which the bloodstream isemphasized relatively to still objects is obtained. This MRA suppressesfat by applying a fat suppression pulse as a pre-pulse immediatelybefore application of a flip-pulse.

[0005] To narrow the band of the fat suppression pulse, as describedabove, the pulse width of the fat suppression pulse must be increasedfrom, e.g., ±π to ±4·π. A very long standby time is required afterapplication of a pre-pulse to application of a flip-pulse. In manycases, the application period (repetition time TR) of the flip-pulsemust be set 2 to 3 times that used when no pre-pulse is applied.

[0006] The photographing time required to acquire all data necessary forimage generation increases several times. This causes various clinicalproblems: photographing cannot be completed within one breath-holdingtime, the burden on a patient increases, and the generation of motionartifacts increases.

BRIEF SUMMARY OF THE INVENTION

[0007] It is an object of the present invention to greatly shorten thephotographing time while ensuring a necessary image contrast in amagnetic resonance diagnostic apparatus using a pre-pulse such as a fatsuppression pulse.

[0008] A magnetic resonance imaging apparatus applied flip-pulses andgradient magnetic field pulses to generate a plurality of MR signals andto add encoding information to the MR signals, applies pre-pulses toobtain a predetermined effect, and generates an image on the basis ofthe MR signals. The MR signals include first MR signals arranged in afirst region in a k-space and second MR signals arranged in a secondregion in the k-space. An applying condition for the second MR signalsis differed from that for the first MR signals. The applying conditionis defined as a ratio of the number of the pre-pulses to the number ofthe flip-pulses applied in an unit time. The pulse sequence time can beshortened while good image contrast is ensured.

[0009] Additional objects and advantages of the invention will be setforth in the description which follows, and in part will be obvious fromthe description, or may be learned by practice of the invention. Theobjects and advantages of the invention may be realized and obtained bymeans of the instrumentalities and combinations particularly pointed outhereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

[0010] The accompanying drawings, which are incorporated in andconstitute a part of the specification, illustrate presently preferredembodiments of the invention, and together with the general descriptiongiven above and the detailed description of the preferred embodimentsgiven below, serve to explain the principles of the invention.

[0011]FIG. 1 is a block diagram showing the arrangement of a magneticresonance imaging apparatus according to an embodiment of the presentinvention;

[0012]FIG. 2 is a timing chart showing an example of a pulse sequencewhen one pre-pulse is applied for one flip-pulse in the embodiment;

[0013]FIG. 3 is a timing chart showing an example of a pulse sequencewhen one pre-pulse is applied for two flip-pulses in the embodiment;

[0014]FIG. 4 is a timing chart showing an example of a pulse sequencewhen one pre-pulse is applied for three flip-pulses in the embodiment;

[0015]FIG. 5 is a timing chart showing an example of a pulse sequenceapplied to a 3 DFT method in the embodiment;

[0016]FIG. 6 is a view showing changes in the pre-pulse applicationfrequency in the k-space of the 3 DFT method in the embodiment; and

[0017]FIG. 7 is a view showing changes in the pre-pulse applicationfrequency in the k-space of a 2 DFT method in the embodiment.

DETAILED DESCRIPTION OF THE INVENTION

[0018] A magnetic resonance imaging apparatus according to an embodimentof the present invention will be described in detail below withreference to the several views of the accompanying drawing.

[0019]FIG. 1 is a block diagram showing the arrangement of a magneticresonance imaging apparatus according to an embodiment of the presentinvention. For example, a cylindrical static magnetic field magnet 1incorporates a shim coil 3 for receiving power from a shim coil power 6to generate a magnetic field for enhancing the uniformity of a staticmagnetic field under the control of a computer unit 12, a gradient coil2 for generating respective gradient magnetic field pulses for threeorthogonal axes, and a probe (RF coil) 4 for applying an RF magneticfield pulse (RF pulse) to an object to be examined and receiving an MRsignal from the object. Note that the probe 4 may not be for bothtransmission and reception, but transmission and reception probes may beseparately arranged.

[0020] A sequence control unit 10 controls in accordance with apredetermined pulse sequence a gradient coil power 5 for applying agradient magnetic field pulse to an object to be examined via thegradient coil 2, a transmitter 7 for applying an RF magnetic field pulseto the object via the probe 4, and a data acquisition unit 11 foracquiring an MR signal received via the receiver 9. The computer unit 12functions as the host computer of the whole apparatus, and in additionhas an arithmetic function of generating an MR image by a 2 DFT(2-Dimensional Fourier Transformation) method or a 3 DFT (3-DimensionalFourier Transformation) method on the basis of MR signals acquired bythe data acquisition unit 11. A display 14 displays an MR imagegenerated by the computer unit 12, and various pieces of information.

[0021] A pulse sequence executed under the control of the sequencecontrol unit 10 is set to a specific pulse sequence among an SE (SpinEcho) method, FE (Field Echo) method, and EPI (Echo Planar Imaging)method, or arbitrarily selected from them by the operator via a console13. A pre-pulse such as a fat suppression pulse can be inserted in thispulse sequence, and the pre-pulse application method can be controlledby the sequence control unit 10. The pre-pulse application method is acharacteristic feature of the present invention, and will be describedin detail. The pre-pulse includes in addition to the fat suppressionpulse a pre-saturation pulse, tagging pulse, MTC (Magnetization TransferContrast) pulse for visualizing mainly the peripheral blood vessel of abrain parenchyma or the like by light contrast, and IR (InversionRecovery) pulse. The present invention can be applied to any of thepulses.

[0022]FIG. 2 shows a pulse sequence when a spin warp method and 3 DFTmethod are respectively adopted as an MR signal generation technique andvisualization technique, and one pre-pulse is applied every time oneflip-pulse (exciting RF pulse) is applied. In this case, the applyingcondition is 1. The applying condition is defined as a ratio of thenumber of the pre-pulses to the number of the flip-pulses applied in anunit time(one second).

[0023] As is well known, the 3 DFT method uses a gradient magnetic field(readout gradient magnetic field: RO) along a given axis among threedifferent gradient magnetic field pulses along gradient axes generatedby the gradient coil 2 in order to encode the frequency of an MR signal,uses a gradient magnetic field (slice selective gradient magnetic field:SLICE) along another axis in order to select a slice and encode theslice of the MR signal, and uses a gradient magnetic field (phaseencoding gradient magnetic field: PE) along the remaining axis in orderto encode the phase of the MR signal. Note that the 2 DFT method(2-Dimensional Fourier Transformation method) uses the gradient magneticfield (SLICE) not for slice encode but only for slice selection, unlikethe 3 DFT method.

[0024] In the 3 DFT method, the flip-pulse is repetitively applied inthe period of a repetition time TR while the patterns of slice encodeand phase encode are slightly changed. In the case of FIG. 2, onepre-pulse is applied for each flip-pulse.

[0025]FIG. 3 shows a pulse sequence when one pre-pulse is applied everytime two flip-pulses are applied (the applying condition is 0.5). Inthis case, a pre-pulse is applied in correspondence with one of a pairof adjacent flip-pulses, but no pre-pulse is applied for the otherflip-pulse.

[0026]FIG. 4 shows a pulse sequence when one pre-pulse is applied everytime three flip-pulses are applied(the applying condition is ⅓). In thiscase, a pre-pulse is applied in correspondence with one of threesuccessive flip-pulses, but no pre-pulse is applied for the remainingtwo flip-pulses.

[0027]FIG. 5 shows a 3 DFT pulse sequence according to the embodiment.FIG. 6 shows the relationship between a plurality of regions in thek-space and the pre-pulse application frequency of each region. As shownFIG. 2, one pre-pulse is applied for one flip-pulse in a period (firstperiod) in which MR signals in the first region (central region)centered on zero encode in the k-space that most influences the imagecontrast are acquired.

[0028] As shown in FIG. 3, one pre-pulse is applied for two flip-pulsesin a period (second period) in which MR signals in the second regionaround the first region are acquired.

[0029] As shown in FIG. 4, one pre-pulse is applied for threeflip-pulses in a period (third period) in which MR signals in the thirdregion around the second region are acquired.

[0030] No pre-pulse is applied for a flip-pulse in a period (fourthperiod) in which MR signals in the outermost region (fourth region)around the third region that least influences the image contrast areacquired. In angiography, the contrast immediately after the start ofphotographing is important. For this reason, application of a pre-pulseand flip-pulse starts after an idle time.

[0031] In this way, the pre-pulse application frequency with respect tothe flip-pulse, i.e. the applying condition, is slightly decreasedoutward from zero encode in the k-space, and no pre-pulse is applied inthe outermost region, i.e., the pre-pulse application frequency withrespect to the flip-pulse is changed every period on the photographingtime axis. In other words, the pre-pulse application interval is changedover time on the photographing time axis. This can greatly shorten thephotographing time, compared to a case wherein one pre-pulse is appliedfor one flip-pulse uniformly in all the regions of the k-space. Sincethe pre-pulse frequency is decreased outward from the central regionwhich most influences the image contrast, a remarkable decrease in imagecontrast can be suppressed.

[0032] As shown in FIG. 7, the present invention can also be applied tothe 2 DFT method. More specifically, one pre-pulse is applied for oneflip-pulse in a period in which MR signals in the central region (firstregion) of the k-space that most influences the image contrast areacquired. one pre-pulse is applied for two flip-pulses in a period inwhich MR signals in the intermediate region (second region) areacquired. One pre-pulse is applied for three flip-pulses in a period inwhich MR signals in the outer region (third region) are acquired. Nopre-pulse is applied for a flip-pulse in a period (fourth period) inwhich MR signals in the outermost region (fourth region) are acquired.Also in the 2 DFT method, the pre-pulse application frequency withrespect to the flip-pulse is slightly decreased outward from zero encodein the k-space, and no pre-pulse is applied in the outermost region.Accordingly, the 2 DFT method can attain the same effects as those ofthe 3 DFT method.

[0033] As has been described above, pre-pulses are applied at a highfrequency in a region (first region) near zero encode in the k-spacethat highly influences the image contrast. As the region is farther fromzero encode, the application frequency is decreased more. For the wholepulse sequence, the number of applied pre-pulses greatly decreases incomparison with a case wherein one pre-pulse is applied for oneflip-pulse at a conventional fixed frequency. Along with the decrease,the photographing time can be greatly shortened. Since the frequency isset high near zero encode, a necessary image contrast can be ensured.

[0034] The present invention is not limited to the above-describedembodiment, and can be variously modified. For example, in the abovedescription, the pre-pulse frequency with reference to the flip-pulse isdecreased to one pre-pulse for one flip-pulse, one pre-pulse for twoflip-pulses, and one pre-pulse for three flip-pulses. However, thepresent invention is not limited to this. The frequency may be decreasedstepwise to one pre-pulse for one flip-pulse, one pre-pulse for threeflip-pulses, and one pre-pulse for five flip-pulses, or may be changedin various patterns.

[0035] A plurality of frequency pattern data may be prepared in thememory of the sequence control unit 10, and may be selectively used inaccordance with an instruction from the operator. Further, variouspatterns can be adopted for the division method of regions usingdifferent frequencies in the k-space, i.e., for region boundaries. Aplurality of region division pattern data in the k-space may be preparedin the memory of the sequence control unit 10, and may be selectivelyused in accordance with an instruction from the operator. Thesefrequency patterns and region division patterns can be arbitrarilycombined and used to arbitrarily adjust the image contrast and theoutput of artifacts.

[0036] Additional advantages and modifications will readily occur tothose skilled in the art. Therefore, the invention in its broaderaspects is not limited to the specific details and representativeembodiments shown and described herein. Accordingly, variousmodifications may be made without departing from the spirit or scope ofthe general inventive concept as defined by the appended claims andtheir equivalents.

What is claimed is:
 1. A magnetic resonance imaging apparatus forapplying flip-pulses and gradient magnetic field pulses to generate aplurality of MR signals and to add encoding information to the MRsignals, applying pre-pulses to obtain a predetermined effect, andgenerating an image on the basis of the MR signals, wherein the MRsignals include first MR signals arranged in a first region in a k-spaceand second MR signals arranged in a second region in the k-space, anapplying condition for the second MR signals is differed from that forthe first MR signals, the applying condition is defined as a ratio ofthe number of the pre-pulses to the number of the flip-pulses applied inan unit time.
 2. An apparatus according to claim 1 , wherein the firstregion includes a region centered on zero encode in the k-space, and thesecond region includes a region outside the first region.
 3. Anapparatus according to claim 2 , wherein the ratio for the first MRsignals is higher than that for the second MR signals.
 4. An apparatusaccording to claim 1 , wherein the second MR signals are generated underthe applying condition that no pre-pulse is applied.
 5. An apparatusaccording to claim 2 , wherein the first MR signals are generated underthe applying condition that the ratio is 1, the second MR signals aregenerated under the applying condition that the ratio is 0.5 or lessthan.
 6. An apparatus according to claim 2 , wherein the MR signalsinclude third MR signals arranged in a third region outside the secondregion, the first MR signals are generated under the applying conditionthat the ratio is 1/L, the second MR signals are generated under theapplying condition that the ratio is 1/M (M>L), the third MR signals aregenerated under the applying condition that the ratio is 1/N (N>M). 7.An apparatus according to claim 6 , wherein the third MR signals aregenerated under the applying condition that no pre-pulse is applied. 8.An apparatus according to claim 1 , wherein a combination of theapplying condition for the first signals and the applying condition forthe second signals can be changed.
 9. An apparatus according to claim 1, wherein a boundary between the first and second regions in the k-spacecan be changed.
 10. An apparatus according to claim 1 , wherein thepre-pulse includes a fat suppression pulse, a pre-saturation pulse, anMTC pulse, and an IR pulse.
 11. A magnetic resonance imaging apparatusfor applying flip-pulses and gradient magnetic field pulses to generatea plurality of MR signals and to add encoding information to the MRsignals, applying pre-pulses to obtain a predetermined effect, andgenerating an image on the basis of the MR signals, wherein an applyingcondition in a first period on a pulse sequence time axis is differedfrom that in the second period, the applying condition is defined as aratio of the number of the pre-pulses to the number of the flip-pulsesapplied in an unit time.
 12. An apparatus according to claim 11 ,wherein first MR signals in a first region centered on zero encode in ak-space are acquired in the first period, and second MR signals in asecond region outside the first region are acquired in the secondperiod.
 13. An apparatus according to claim 12 , wherein the applyingcondition in the first period is higher than that in the second period.14. An apparatus according to claim 11 , wherein no pre-pulse is appliedin the second period.
 15. An apparatus according to claim 12 , whereinthe applying condition in the first period is 1, the applying conditionin the second period is 0.5 or less than.
 16. An apparatus according toclaim 12 , wherein the applying condition in the first period is 1/L,the applying condition in the second period is 1/M (M>L), the applyingcondition in a third period other than the first and second periods is1/N (N>M).
 17. An apparatus according to claim 16 , wherein no pre-pulseis applied in a period other than the first, second, and third periods.18. An apparatus according to claim 11 , wherein a combination of theapplying condition in the first period and the applying condition in thesecond period can be changed.
 19. An apparatus according to claim 12 ,wherein a boundary between the first and second regions in the k-spacecan be changed.
 20. A magnetic resonance imaging apparatus forrepetitively applying flip-pulses, receiving a plurality of MR signalsencoded in accordance with a 2 DFT or 3 DFT imaging method after therespective flip-pulses are applied, and generating an image on the basisof the MR signals, wherein an application interval of a pre-pulse ischanged between first and second periods on a pulse sequence time axis.